1. Field of the Invention
The present invention relates to a threshold matrix used for generation of a halftone dot image representing a grayscale original image.
2. Description of the Background Art
To create data for printing plate from data of an original image of gray scale (i.e., continuous tone), halftone dots are used in many cases. In a generally-used AM (Amplitude Modulated) screening, grayscale representation is made by changing the size of dots, instead of changing the number of dots constituting a halftone dot image, but since the dots are regularly arranged, when a periodic image is created by using halftone dots, there arises moire in the created halftone dot image. Then, techniques for reducing an effect of moire have been disclosed in, e.g., U.S. Pat. No. 6,128,099 (Document 1) where a plurality of dot centers are arranged randomly and U.S. Pat. No. 5,859,955 (Document 2) where a plurality of highlight-side dot centers and a plurality of shadow-side dot centers are arranged while being stochastically distributed.
On the other hand, in an FM (Frequency Modulated) screening, grayscale representation is made by changing the number of dots of certain size, which are arranged appropriately (in a random fashion without extreme local difference in density).
In general, however, by changing a halftone dot area around the highlight-side dot center in accordance with the variation in gray level on the highlight side of an original image and changing the halftone dot area around the shadow-side dot center in accordance with the variation in gray level on the shadow side thereof, the variation characteristics of halftone dots in accordance with the variation in gray level on the highlight and shadow sides of the original image can be symmetrical to each other and therefore a preferable halftone dot image can be formed.
In the technique of Document 1 where the dot centers are arranged randomly, usually, there is a great variation in the distance between adjacent dot centers and depending on the method of growing the halftone dots, the dots whose dot centers are positioned near are joined quickly and the dots whose dot centers are positioned far are joined slowly, and especially, there arises a variation in joint of the halftone dots representing a concentration of about 50%, and this causes graininess in a printed matter. Also in the technique of Document 2 where a plurality of highlight-side dot centers and a plurality of shadow-side dot centers are arranged, since the regularity that a plurality of shadow-side dot centers are arranged around one highlight-side dot center is lost, there arises remarkable graininess in a created image. In the FM screening, there also arises graininess due to a varied joint of dots.